FIG. 4 explains the phenomenon of show-through, and it is assumed that the objects shown are colored ones. In accordance with wide spread availability of equipment that can capture or print color images, the use of color images in magazines, catalogues, advertisement, or newspapers has become common. Examples of equipment that can capture color images are color scanners, digital cameras, and digital color copiers. In almost all the cases an image or matter is printed on both sides of the paper. However, sometimes the image on one side of the paper can be seen even from the other side. This fact is shown in FIG. 4. The image 4C is printed on the back side of the paper and the other images are printed on the front side of the paper. However, the image 4C printed on the back side of the paper is seen from the front side. Further, in many cases, a color document has a different background color, or has image patterns, other patterns, and photographs together with one another. Therefore, the problem of removing “show-through” from the input image by performing digital image processing has been regarded as being difficult to handle although it is important for improvement in image quality.
Methods for removing show-through on special equipment, such as a book scanner or a double-side scanner, are known. Such methods have been disclosed, for example, in Japanese Patent Nos. HEI 07-8795A, HEI 08-265563A, HEI 09-205544A, HEI 09-233319A, or U.S. Pat. Nos. 5,973,792 and 5,932,137.
U.S. Pat. No. 5,932,137 discloses a method and apparatus for mitigating the effects of show-through by scanning both sides of the paper and storing the scanned images. The invention according to this publication is realized by scanning a duplexed document with show-through derived from the second side at least on the first side, storing a first side image and a second side image, generating a representation (mirror-image conversion, registration of images) corresponding to a component attributable to show-through from the second side image to the first side, and correcting (using a show-through coefficient) the image by removing the show-through from the first side as a function of the representation of the second side image. Further, the image processing method for mitigating the effects of show-through is performed by scanning a first side and then a second side of a duplexed document with show-through derived from the second side on the first side, storing the first side image and the second side image, generating a representation (mirror-image conversion, registration of images) corresponding to a component attributable to show-through from the second side image to the first side (and from the first side image to the second side), and correcting (using a show-through coefficient) the image by removing the show-through as a function of the representation of the second side image (first side image) from the first side (second side). Further, in this publication, there has been described a document printing system which mitigates the effects of show-through for printing an image with mitigated effects of show-through. More specifically, this system comprises a scanner that scans a duplexed document with show-through derived from the second side at least on the first side, and stores images on the first side and the second side, an image processing circuit (mirror-image conversion, registration of images) that generates a representation corresponding to a component attributable to show-through from the second side image to the first side, and an image processing unit (using a show-through coefficient) that removes the show-through from the first side as a function of the representation of the second side image, and corrects the image.
In the above-mentioned methods, at first, the input images on both sides of the paper are accurately aligned in their position, and the images on both sides are compared to each other to estimate a transmission coefficient of paper. By subtracting a component attributable to show-through from the front side image using the transmission coefficient, the show-through is removed. However, there is a problem in these methods such that the methods can not be applied to any other equipment except specific input equipment that can input and store images on both sides and accurately register the images or such environments.
Other methods for removing the show-through have been proposed based on analysis or binarization of image density using only information for a single-side image. For example, the methods have been proposed in Japanese Patent Nos. HEI 11-187266A, HEI 11-41466A, [J. Sauvola, T. Seppanen, S. Haapakoski, and M. Pietikainen, “Adaptive document binarization,” Proc. 4th Int. Conf. Document Analysis and Recognition (Ulm, Germany) Aug. 18-20, 1997, pp. 142-146.], U.S. Pat. Nos. 5,646,744 and 5,832,137. However, when the image is complicated, it is difficult to discriminate between a front side image and show-through using density or color distribution. Using such analyses, a risk arises such that a character having low contrast to a background in particular, for example, a character in yellow on a white backgrounds, may be processed as a show-through image.
Thus, the method of scanning and storing the images has a problem that it can only be utilized on special equipment and cannot be applied to other equipment. Such special equipment can input and store images on both sides of the paper and can accurately align the positions of both the images.
On the other hand, the method of using the image density has a problem that it is difficult to discriminate between a front side image and show-through image using density or color distribution when the image is complicated. With this method, there may be a case that a character having low contrast to a background in particular, for example, a character in yellow on a white background is processed as show-through.